Process of making arsenate of lead



Patented Mar. 17, 1925.

UNITED STATES PATENT OFFICE.

HENRY HOWARD, OF CLEVELAND, OHIO, ASSIGNOR TO THE GRASSELLI CHEMICAL COMPANY, OF CLEVELAND, OHIO, A CORPORATION OF OHIO. l

PROCESS OF MAKING ARSEN'ATE OF'LEAD.

No Drawing.

To all whom it may concern:

Be it known that I, HENRY HOWARD, a citizen of the United States, residing at Cleveland, in the county of Cuyahoga and 6 State of Ohio, have invented certain new and useful Improvements in Processes of Making Arsenate of Lead, of which the following is a specification.

This invention relates toa process of 10 making arsenate of lead which consists gen- 1 erally in directly combining arsenious oxid (AS203) with the higher oxids of lead in the presence of water and a catalytic agent. Heretofore arsenate of lead has been made by first forming a soluble salt of lead and a soluble salt of arsenic acid and causing them to react insolution or by dissolving lead oxid (PbO) in a solution of arsenic acid. These two processes both involve the preliminary formation of arsenic acid from arsenious oxid, which constitutes the commercial supply of arsenic for the manufacture of lead arsenate. More recently it has been proposed to make arsenate of lead directly from arsenious oxid by several methods, among which may be mentioned the following: (1) Lead oxid (PhD) and arsenious oxid (As O are mixed together and roasted under oxidizing conditions; (2) red lead (1 13 0 and arsenious oxid are roasted together under oxidizing conditions. In this case the excess of oxygen in the red lead over that present in PbO assists the oxidation; (3) lead peroxid (1 b0,) and arsenious oxid are roasted together. In this case the oxygen required for converting the arsenious oxid to the arsenic state is furnished by the lead peroxid; (4) arsenious oxid is roasted with lead nitrate.

After a very extensive research in connection with the manufacture of arsenate of lead, I have developed a successful process Application filed April 23, 1924. Serial Non-08,574.

mixture of red lead and lead peroxide in the proportions indicated in the equation:

or for the manufacture of the di-basic arsenate by the use of lead peroxide alone in accordance with the equation:

Since the procedure is substantially identical in both cases the invention will be illustrated hereinafter by reference to the process of making the di-basic arsenate.

One very important feature of my invention resides in the use of an aqueous reaction medium. It is noted in this connection that while in the prior processes an aqueous medium has been employed in combining lead oxid (P100) and soluble lead salts with arsenic acid and salts thereof, the dry process has been employed exclusively in the manufacture of arsenate of lead directly from arsenious oxid. I have found that for directly combining the higher oxids of lead with arsenious oxid the best results are obtained when an aqueousreaction medium is employed. The reaction may be carried out either in the presence of only suflicient water to form a thick paste or in the presence of sufficient water to form a slurry or suspen sion. The former method is applicable and may be preferred when working with relatively pure grades of oxids of lead, the latter method being preferred with impure grades. Both methods are applicable with both the pure and the impure grades of the oxids of lead, but the latter method gives a better conversion with the impure oxids than the former. The purity of the arsenious oxid used has no appreciable elfect, ordinary commercial giving satisfactory results.

I have also found that the quantity of catalytic agent used has a decided influence upon the success of the process. In this connection it may be stated that the action of the catalytic agent does not appear to be a truly catalytic action as is evidenced by the fact that the use of an amount of the catalyst either more or less than the optimum amount results in a decrease in the conversion and that under such circumstances the conversion cannot be improved by prolonged treatment.

action requirin The preferred catalyst is nitric vacid and the optimum amount thereof is about 1 to 2% by weight of the combined weights of the oxid or oxids of lead and arsenious oxid in the reaction mixture.

The best results have been obtained by using about 1.4 parts b weight of nitric acid to each 100 parts y weight of lead peroxid. The use of as much as 2 parts and as little as .6 parts by weight of nitric 'acid to each 100 parts by weight of lead which thereaction begins is reached, and.

a considerable amount of heat is evolved. With some pure grades of lead peroxid the reaction be ins at a temperature of about 50 C. whi e with impure grades it may be necessar'y to heat up to'90 O. or higher to initiate the reaction; As stated, the reaction takes place very qulckly with a spontaneous increase in the temperature of the reactlon mixture which vanes from about.

10 C. for an impure grade of oxid to as high as 50 C. for a pure grade of oxid. During the reaction the 'mass ufls up and becomes very voluminous. T e course of the reaction is indicated by the rise in the temperature of the reaction mixture and by its change in color and physical condi- -t 1on. For quick work, and particularly when working with commercial oxids of lead which are not of high purity, I prefer to carry out the reaction by heating the reaction mixture in an autoclave under steam prelslsureup to say 40-50 pounds per square inc The productof. the reaction which, is in the form of a White paste may be transferred directly to a drier and then, after bein dried, to a ulverizer, or if a pure pro net is desired: the reaction product may be first washed. with water and then dried and pulverized.

The method according to which an aqueous slurry or suspension is used is somewhat slower than the paste method described above, the completion of the reseveral minutes. This method vis apphcable in the use of either pure or im ure oxids of lead, but is preferred in t e use of the im ure oxids becauseit 'ves' a considerably better conversion, t 9 pure oxids of lead giving almost perfect conversions by either the pasteor the slurry method.

In carrying out the slur method an iaxample of a preferred procedure is as folows:

100 pounds of lead peroxid, 41 pounds of arsenious oxid, 2 pounds of nitric acid (100%) and 300 pounds of water are mixed and the mixture continuously stirred and heated up to a reaction temperature which varies from about 50 C. in the case of a pure lead peroxid to as high as 90 C. or

igher in the case of an impure oxid. After the reaction temperature is reached the re action is completed in a-few minutes, the end of the reaction being indicated by the color of the mixture and by the fact that no further rise in temperature occurs. The lead arsenate product is separated from the resulting slurry by setthng or filtration and dried and ground, or first washed and then dried and ound in the usual way.

With pure 'OXldS of lead the product is white or light gray and an analysis thereof shows a conversion as high as 99%. The ratio of .PbO to As O in the product de termined by analysis usually runs a little higher than the theoretical ratio, for instance the ratio shown by the analysis may be about,1.97 whereas the theoretical ratio is 1.94 .for' PbI-IAsO This indicates the With some impure oxids of lead the conversionmay run as low as 90% and the product may be of a brownish color due apparently to the presence of lower oxids of lead. In all cases, however, the soluble arsenic content is very low, being in the neighborhood of 0.5%. V

In the foregoing description oxids of lead running in the neighborhood of 98% or higher are referred to as oxids of high urity, while the references to oxids of ower grade or purity embrace oxids runnin from say 95% downward.

T e slurry methodin addition to being better applicable to impure oxids than the paste method, has the further advantage that it gives a more flufly and voluminous product. p 4

In conclusion it is noted that my wet method of combining arseni'ous oxid and higher oxids of lead to form arsenate of lead is simple and quick and avoids both which comprises heating to reacting temperature a mixture containing arsenious oxid, a higher oxid of lead, a catalyst, and water in quantity at least suflicient to form a stiff paste.

2. Process as defined in claim 1 in whichthe higher oxid of lead is lead peroxid.

3. Process as defined in claim 1 in which the catalyst is nitric acid and the quantity thereof in the mixture is equal to from 1 to 2% of the combined weights of the higher oxid of lead and arsenious oxid.

4. Process as defined in claim 1 in which the' quantity of water in the mixture is suflicient to form a slurry with the solid components.

- and heating the mixture to a latter in quantity equal to about 1.4%

of the weight of the oxid of lead, and water in quantity suflicient to produce a slurry,

temperature of from 50 C. to 100 C.

In testimony whereof, I aifix my signature.

HENRY HOWARD. 

